The present invention relates to Code Division Multiple Access (CDMA)-based communication systems and in particular to an arrangement and a method to reduce the amount of handover-related signalling traffic that must be sent between user equipment and the network.
In Code Division Multiple Access (CDMA) communication systems, a plurality of base stations cover each a certain geographic area and provide communication services to users within such areas. Characteristic for CDMA-systems is that the radio spectrum is shared between the different users by assigning specific and unique code sequences to the uplink and downlink channels between base station and user equipment. These code sequences are used by the transmitters to transform signals into wideband spread spectrum signals. In the receiver units, base station or user equipment, said wideband signals from a specific transmitter are retransformed into the original bandwidth by using the same code sequence as that transmitter while signals marked with different codes remain wideband signals and, thus, are interpreted by the receiver as part of the background noise.
An aspect specific to the multiple access technique used in CDMA relates to the fact that all users transmit wideband signals potentially at the same time and using the same frequency spectrum. Thus, one limiting factor in CDMA-based systems is a decreasing signal-to-noise ratio in the communication channels due to an increasing interference level caused by other transmitters. Said signal interference level per cell results from transmitters within said cell as well as from transmitters in the neighbouring cells. A cell in a CDMA-based communication system is not a strictly defined area but merely do cells partly overlap each other at cell borders and, thus, do not form sharp cell borders. This is obvious from the fact that the range of a cell essentially depends on parameters as, e.g., the downlink power level of the radio base station, the number of user equipments in the cell, and environmental effects causing, e.g., fading effects. In particular, a CDMA-based communication system also allows different layers of cells that cover the same geographic area.
From this it becomes apparent that it is a normal case for a user equipment in a CDMA-based communication system to have simultaneous connections to more than one radio base station. With regard to capacity and quality of services, the user equipment must have established radio connections to the best cells, i.e. to the radio base stations that are best according to a certain decision criterion. Therefore, a user equipment must keep track on a number of cells in order to be able to perform the necessary handovers and to achieve optimised communication conditions. This is done by means of a set of measurement cells containing at least all cells from which the user equipment receives a signal. Those cells to which the user equipment has an established connection are contained in the active set, which forms a subset of said measurement set.
It becomes also apparent that the specific kind of cell structure as described above leads to specific requirements in handover situations which is the object of the present invention.
The present invention relates to an arrangement and a method to render the handover-related signalling traffic in a CDMAbased communication system more effective.
It is a first object of the present invention to achieve a handover method for user equipments that reduces the signalling traffic when performing a handover.
It is in particular an object of the present invention to achieve a handover method that is applicable in case of oscillating handovers between two cells in a CDMA-based communication system.
It is another object of the present invention to achieve a handover method that allows a faster performance of handovers.
It is still another object of the present invention to achieve a handover method that can reduce the amount of signalling information that must be transmitted when performing a handover.
It is yet another object of the present invention to achieve a handover method that can reduce the number of handover decisions.
It is yet another object of the present invention to achieve a user equipment that has established communication links to the best cells in accordance with decision criterions that are applied by both user equipment and an appropriate unit in the network.
Briefly, these and other objects of the present invention are accomplished by the arrangement and method according to the present invention. Said arrangement comprises, i.a., a delay unit that can be implemented solely in the user equipment or in both user equipment and an appropriate unit in the network, e.g., in the radio network controller or the radio base station. The delay unit indicates a delay period for cells that shall be removed from the measurement set of a user equipment. Said method comprises, i.a., the step of releasing a communication link to the radio base station of a cell, i.e. removing its cell identity from the active set to the measurement set of a user equipment, while keeping information about its neighbour cells, which have no other neighbour cell in the modified active set, for a time period that is indicated by said delay unit. Information about said neighbour cells can be kept, e.g., in the measurement set. The neighbour cells of a cell that shall be included in the active set are only retrieved if this cell shall be included for the first time in an oscillating phase or if the delay period has expired. A delay unit that is implemented in both user equipment and an appropriate unit in the network saves for the user equipment the additional step of retrieving neighbour cells for which a delay period has expired because the network is aware of this fact and will send said neighbour cells automatically.
As a first advantage, the present invention allows to reduce the amount of signalling traffic due to the fact that neighbour cells are only sent when a cell is included in the active set for the first time in an oscillating phase or if a delay period has expired.
It is thus an advantage that information about neighbour cells of a cell to which a user equipment intends to establish a communication link need only to be retrieved once from the network when performing an oscillating handover.
It is another advantage that the number of handover decisions can be reduced.
It is still another advantage of the present invention that the user equipment will have established communication links to the best cells in accordance with decision criterions that are applied by the user equipment and an appropriate unit in the network.
Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.